Abstract

High field pinning centers in MgB2 doped with 2 at. % carbon under a low and a high hot isostatic pressures have been investigated by transport measurements. The field dependence of the transport critical current density was analyzed within the different pinning mechanisms: surface pinning, point pinning, and pinning due to spatial variation in the Ginzburg-Landau parameter (Δκ pinning). Research indicates that a pressure of 1 GPa allows similar pinning centers to Δκ pinning centers to be obtained. This pinning is very important, because it makes it possible to increase the critical current density in high magnetic fields at 20 K and 25 K. Our results indicate that the δT c and δl pinning mechanisms, which are due to a spatial variation in the critical temperature (T c) and the mean free path, l, respectively, create dislocations. The high density of dislocations with inhomogeneous distribution in the structure of the superconducting material creates the δl pinning mechanism. The low density of dislocations with inhomogeneous distribution creates the δT c pinning mechanism. Research indicates that the hot isostatic pressure process makes it possible to obtain a high dislocation density with a homogeneous distribution. This allows us to obtain the δT c pinning mechanism in MgB2 wires. In addition, a high pressure increases the crossover field from the single vortex to the small vortex bundle regime (B sb) and improves the δT c pinning mechanism. Our research has proved that a high pressure significantly increases the crossover field from the small bundle to the thermal regime (B th), with only a modest decrease in T c of 1.5 K, decreases the thermal fluctuations, increases the irreversibility magnetic field (B irr) and the upper critical field (B c2) in the temperature range from 4.2 K to 25 K, and reduces B irr and B c2 above 25 K.